The Right Optics Concept for the Right Dimension of the High Luminosity LHC Project

While the LHC optics is approaching its limit in terms of beta*, not only bounded, somehow in relative, by the aperture of the existing inner triplets, but also, in absolute, by the match-ability, the flexibility and, at some point (not yet reached) the chromatic aberrations of the corresponding collision optics, the High Luminosity LHC project requests to triple or even quadruple this effort in order to meet the challenging performance target of 250 1/fb per year. This exigency, or even only half of it, cannot be satisfied without a complete change of point of view in the way low-beta* optics are thought and realized. After a long introduction presenting the main objectives and strategies of the HL-LHC project, this article will summarize the standard squeezing optics techniques and chromatic correction strategy presently used in the LHC, and will remind the main optics limitations resulting in this context. Identified a few years ago in the framework of the former upgrade project, these limitations then led to the establishment of the Achromatic Telescopic Squeezing scheme as a key solution to the various issues, which has been a formidable background for the overall HiLumi Design Study, and stays an in-built feature of the baseline HL-LHC configuration and of any of its possible variants.